Many people say No$GBA but I recommend you to have both emulators because some games doesnt run No$GBA.. I will not name which games but I am telling you to use both emulators. I have the latest version of No$gba 2.6a and the latest newest DeSmuMe. They are both an excellent emulators!
I have DeSmuMe and it is very fast emulator at least for my 2 years old computer. Some people run it slow but you have to understand DeSmuMe:
What are the recommended hardware requirements for DeSmuME?
The following configuration should assure that almost every game can be played at full speed:
* Windows OS: Windows XP or newer
* Linux OS: Any recent Linux distribution with a 2.6 kernel.
* 3 GHz Core2 Duo processor or better
* 512 MB Available RAM
* Available Hard Drive Space: A few MB for DeSmuME, 100MB or so for a bunch of savestates and savefiles.
* Sound Card.
* Microphone.
In general, the better a CPU, the better the performance. The faster the memory, the better. DeSmuME benefits a little but from multiple cores but not as much as it does individual cores being faster. DeSmuME is slower than no$gba, and is likely always to be.
You should get No$GBA 2.6a (The latest one) and DeSmuMe 0.9.6 (with latest svn version). ENJOY!